Interpretive Summary: Arid and semi-arid regions account for approximately one-third of the land mass of earth. These regions are experiencing continued pressure from population growth in many parts of the world. Water is a critical resource in these regions and is often in short supply and land surface moisture measurements are central to our understanding of the earth’s water system. Currently, there exists no in-situ network capable of estimating wide-area soil moisture. In this paper, we explore an alternative method of estimating soil moisture using U.S. National Lightning Data Network (NLDN) and via the effect soil moisture has on normal broadcast radio waves. It was shown that changes in model estimated soil moisture is related to changes in properties of lightning sensed using data from the NLDN sensor at Lordsburg, NM over the summer of 2005. In the second method with an AM radio transmitter in Tucson, Arizona and two sensors in the San Pedro Basin, it was shown that changes in the radio signal between the two sensors were well-correlated with changes in soil moisture. The long-term goal of this project is to use existing infrastructure of Lightning Detection Networks to detect, locate, and quantify both long-term and short-term changes in soil moisture over North America with a spatial resolution of about 30 km.

Technical Abstract:
Land surface moisture measurements are central to our understanding of the earth’s water system, and are needed to produce accurate model-based weather/climate predictions. Currently, there exists no in-situ network capable of estimating wide-area soil moisture. In this paper, we explore an alternative method of estimating soil moisture through the effect that soil moisture has on electrical conductivity of the soil and on the resulting surface propagation of low-frequency (LF) and medium-frequency (MF) electromagnetic waves (EW). Two sources of EW in these frequencies were explored. The first source is the risetime of lightning generated broadband EW as measured by the U.S. National Lightning Data Network (NLDN). The use of the NLDN sensors can also be extended to include monitoring the signal attenuation and phase changes of anthropogenic radio transmissions. It was shown that changes in land surface model (LSM) estimated soil moisture (from NARR) is related to changes in the average risetime of lightning waveforms using data from the NLDN sensor at Lordsburg, NM over the summer of 2005. The second method uses narrow-band anthropogenic radio transmissions. Using an 830 kHz AM transmitter in Tucson and two sensors in the San Pedro Basin, it was shown that changes in the attenuation between the two sensors were well-correlated with changes in soil moisture.